This invention relates to the design of gas turbine engines, and more particularly to the characterisation of bird impacts, which are a particular problem when such engines are used in aircraft.
Bird impacts can be very damaging to gas turbine engines, and it is necessary to design engines so that they can be safely run on for a period of time after an impact or, at the very least, safely shut down without causing danger to the aircraft or passengers.
The basic techniques for carrying out bird ingestion tests on gas turbine engines are well known, and need not be discussed in detail here.
In such a test, a soft body is fired at a known position on the inlet of the engine. The soft body may be either a dead bird or an artificial soft body designed to replicate closely the impact behaviour of a real bird. Such artificial soft bodies are well known. In the following description, the term “bird” can generally be taken to refer either to a real bird or to an artificial soft body used in a test in place of a real bird. After an impacting bird first hits the structure of the engine (whether static or rotating structure) it will fragment. Damage may be caused to the downstream parts of the engine by these fragments, and the size and trajectories of the fragments will determine the extent of this damage. Normally, the fragmentation can be assessed by viewing photographs or video of the first impact. The observed fragmentation can then be used to model the behaviour of the different parts of the engine following an impact, and thereby to design the engine to minimise damage from bird strikes.
However, there is a problem when an engine is fitted with an obscuring intake. Such intakes are commonly used on military engines, so that the structure of the intake shields the hot components of the engine and prevents its detection by radar or other systems. The problem is that an incoming bird will strike the surface of the intake and will fragment, but the fragments are then hidden within the intake and so it is not possible to assess their size and trajectories. To obtain an initial bird condition for analysis, it is necessary to perform a separate impact test for each impact condition. This is time-consuming and expensive.
Previously, impact analyses have been undertaken using a whole, intact bird, a bird fragment or a non-validated slurry.
For the analysis of the bird strike capabilities of engines that will in use be shielded by an obscuring intake, it is vital that the initial conditions of the bird, which are used for analysis, reflect reality. Following a light impact of a bird on the intake, the bird can be modelled as a single fragment whose mass corresponds to that of the largest bird fragment. Following a severe impact, the bird can be modelled as a slurry. In between these two extremes of impact, the bird can be modelled as a combination of fragment(s) and slurry.
United Kingdom patent application GB0807482.5 describes a method for characterising the fragmentation of a soft body following impact with a surface, and its teaching is incorporated herein by reference.
One embodiment of the invention described in GB0807482.5 uses an apparatus 12 as shown in FIG. 1. A gun 14 of known type fires a bird (not shown) along a trajectory 16 to impact on a surface 18 of a target 20. The angle of impact of the bird on the surface 18 is θ. The fragmented bird is deflected off the surface 18 towards a soft-recovery system 22. This comprises a number of large plastic sacks 24 freely suspended on two parallel ropes 26 (only one is visible). The ropes 26 are rigidly fixed at their ends to structure 28. In this embodiment, ten sacks 24 are used, but any suitable number may be employed. The sacks 24 absorb energy from the bird fragments without damaging them, allowing them to fall on to a sheet 30 on the floor for collection and analysis.